Gallium nitride "GaNt" Aluminum nitride "AlN" and Indium nitride "INN" are known as semiconductor compounds of large direct energy gaps. As such they are important electronic materials.
AlN, in the form of ceramic substrate, is applied in high power electronic applications, because of its high heat conductivity, thermal expansion co-efficient close to that of silicon, and good stability at high temperatures.
It has long been known that among the nitrides of group III metals, GaN has potentially the best useful properties as a semiconductor device. Specifically, GaN has semiconducting properties for temperatures up to 600.degree. C. as compared to silicon semiconductor with temperature stability of up to 120.degree. C. The temperature stability and large energy gap of GaN can provide many new high temperature applications for electronic products.
A second important characteristic is that a GaN p-n Junction light emitting diode ("LED") emits visible blue light with a wavelength of approximately 450 nm. GaN has a high efficiency of radiative recombination, and low dislocation mobility. The other semiconductors which are known to emit light in that band are silicon carbide (SIC) and generally A.sup.II B.sup.VI semiconductors such as ZnSe and CdF.sub.2. However, because it is an indirect bandgap material, the luminous efficiency of SiC is only about 0.04 lumen/watt. The A.sup.II B.sup.VI are known to have high defect mobilities and dislocation densities, which reduce their useful life and the power level at which they can operate. In contrast, it is anticipated that LED's made from GaN would have a luminous efficiency of about 0.6 lumen/watt, and remain extremely stable over time.
Thus-GaN and other group III metal nitrides are viable candidates for applications in short wavelength optoelectronics, blue laser systems, full color display systems and high temperature electronics.
Despite their many advantages, nitrides of group III metals including GaN have not been used extensively because of the many difficulties involved in growing such nitrides in bulk crystals. Their thermodynamic properties preclude the standard techniques for the growth of bulk single crystals, appropriate for commercial use. For instance, the high melting temperature and high N.sub.2 pressure at melting, of GaN is in the range where the compound is unstable and readily dissociates. Due to the high melting temperature, the substrate crystals of GaN cannot be obtained by typical crystal growing methods like Czochralski or Bridgman growth from the stoichiometric melts.
Because of the difficulties to produce substances of pure crystalline nitrides of group III metals, the prior art methods use substrates made of materials other than group III nitrides, to develop crystalline nitrides. For example, the nitrides of group III metals like gallium nitride, aluminum nitride, indium nitride or their alloys are deposited on crystalline substrates of different chemical compositions like sapphire or silicon carbide, by Molecular Beam Epitaxy ("MBE") or Metal Organic Chemical Vapor Deposition ("MOCVD").
Specifically atoms of group III metals like gallium and atoms of nitrogen are deposited on a single crystalline substrate by causing them to collide with the substrate. In such known procedures gallium atoms are provided by vaporizing liquid gallium at 1800.degree. C. Nitrogen atoms ere generated from a flow of molecular nitrogen exposed to plasma causing its molecules to dissociate. It is also possible to apply accelerated positive ions by using an electric field for the acceleration to dissociate the nitrogen molecules.
Another prior art method for developing GaN crystal is known as metal organic chemical vapor deposition. Accordingly, the gallium nitride is deposited on a sapphire substrate, by simultaneously applying two chemical reactions: first, decomposing ammonia End second decomposing a metalorganic compound, like trimethylgallium, which is a suitable carrier of gallium. Gallium obtained from the decomposition of the metalorganic compound and the nitrogen derived from ammonia, are deposited on the surface of a sapphire substrate and as a result create a two layer structure. Using a similar method, aluminum nitride deposited on a sapphire substrate has been produced by using trimethylaluminum as a source of aluminum.
Another method for producing gallium nitride crystal is disclosed in the Polish Patent No. 127099. The patent discloses a procedure for crystallization of gallium nitride from a gas phase by sublimation and condensation process under high nitrogen pressure. specifically, according to the disclosed method gallium nitride powder sublimates at temperatures-exceeding 1000.degree. C., at nitrogen pressure higher than 1000 bar. Thereafter, gallium nitride condensation occurs on a sapphire substrate. The temperature difference between the starting material and the substrate would not exceed 500.degree. C.
The procedures disclosed in prior art are therefore mainly limited to the growing of GaN crystal or other group III metal nitride crystals, on a different substrate. Such growth procedures are known as heteroepitaxy production. The gallium nitride structures obtained by such known heteroepitaxy procedures are of low crystalline quality. Their half width at half maximum of the X-ray double crystal reflection curve, known as the rocking curve, is not lower than 200 arcsec, which is not satisfactory for many applications.
One main reason for the poor quality crystals of the prior art is the difference between the lattice constants of the substrates and the deposited layers, which causes a strain field in the structure. The large lattice mismatch, which is 14% for sapphire and 3.4% for SiC substrate, leads to the creation of dislocations, cracking of the layers, island growth and the formation of incoherent boundaries between crystalline grains.
Another difficulty with GaN is its failure to maintain a chemical balance or stoichiometry. Gallium nitride is not stoichiometric because of the high propensity for nitrogen atoms to leave gallium nitride crystals. Therefore, stoichiometric nitrides free of nitrogen vacancies are difficult to obtain. It is commonly believed that the high concentration of nitrogen vacancies is the source of numerous native donor states which are responsible for high free electron concentration observed in group III-nitride semiconductors.
Hence there is a need for multilayer high quality group III metal nitride crystals and consequently n and p type semiconductors derived from such crystals in order to benefit from their potentially important properties.